1. Field of the Invention
This invention relates to an apparatus and method by which streams of gas and liquid may be introduced into a contact zone in a controlled manner in order to facilitate physical or chemical reactions between the two flows. More particularly, this invention relates to the containment of a flame within a surrounding continuous surface of flowing liquid, and to means by which heat from the products of combustion may be transferred to a liquid. Particular applicability is found in generating steam in compact space.
2. Description of the Related Art
It is often desirable to bring together a stream of gas with a stream of liquid in order that a chemical or physical reaction or mass transfer may take place between the two streams. One particular example of such a condition is the contacting of the products of combustion arising from a flame with a liquid, e.g., water, in order to transfer heat to the liquid. Other examples include the contacting of a cooling refrigerant gas with a liquid in order to cool the liquid or the absorption of carbon dioxide or hydrogen sulfide from acid gas by contacting with ethanolamines or gylcols.
When gases and liquids are to be contacted in order to precipitate a chemical or physical reaction, the rate at which the process proceeds depends upon the surface area over which such contacting occurs. In such cases, in order to obtain high rates it is desirable to maximize this contact surface area.
Direct contact heat transfer (DCHT) is the technique by which heating and heated materials are brought into intimate contact with each other without the presence of an intermediate heat transfer surface or barrier. One example of DCHT is in the heating of fluids, primarily water, by the direct contacting of the products of combustion with the liquid. Steam can be raised by spraying water into a stream of hot gases issuing from a burner. Alternatively, hot gas streams generated by combustion can be bubbled through a liquid using submerged combustion heaters to effect heat transfer. Maximization of contact surface area is a desirable objective in each of these cases.
In the case of submerged combustion heaters, the flame or combustion zone is separated from the fluid by a protective tube or cylinder which acts as a shroud or shield. Such shielding elements are often susceptible to severe scaling and corrosion. Similar problems are typical in the case of downhole steam generators for use in injecting heated water or steam into oil-bearing underground formations. The annular metallic sleeves which surround the flame and transport water to the exhaust gas zone in downhole or surface heaters suffer particularly from severe thermal stress related problems of fatigue and cracking.
When a flame is contained within a protective shroud the shroud may on occasion be raised to very high temperatures. Where cooling is not provided the shroud may become glowing hot or even white hot. Where metals are used in such circumstances, cooling is provided to limit rapid deterioration of the metal in the shroud. When cooling, usually in the form of circulating water, is applied to the exterior surface of the flame shroud, steep thermal gradients are formed within the shroud wall. This can lead to metal fatigue and failure of the mechanical integrity of the shroud wall.
The same fatigue and cracking problems arise when unquenched combustion exhaust gases are introduced into contact columns where direct contact cooling occurs. To protect trays and other contact surfaces within such columns, it is often necessary to pre-cool combustion gases.
U.S. Pat. No. 4,604,988 describes an apparatus and method for overcoming such problems, which is of particular applicability to heating water. However, the method as described therein is not of sufficient efficiency for generating steam.
The heavy oils of Western Canada, U.S.A., Venezuela and other parts of the world, and the oil in the oil-sands of Alberta, are too viscous to flow in their naturally occurring state. They need to be heated by some suitable technique in order to make them flow to the producing wells. Over 95% of such heavy oils recovered in the U.S. since 1980 is due to steam injection into these heavy oil formations. Such steam generators transfer heat to the water from the flame and hot combustion gases in the radiant and convection sections of once—through type oil fields stream generators to convert water to steam. It should be noted that the entire tube bundle in the radiant section is exposed to the flame. If the inside tube walls become dry due to water evaporation, hotspots would result eventually causing tube rupture. After transferring their heat in the convection section, the hot gases exit through the stack constituting the major heat loss associated with such steam generators.
The conventional generator looses about 20% of the fuel energy through stack gases, 5-20% through surface piping, and 10% or more in the well bore. Thus, the percentage of energy reaching the oil formation is less than 60% of that in the fuel. The downhole steam generator is located downhole at the sandface and thereby eliminates all the heat losses. This results in 100% of the energy content of the steam and combustion gases being applied to the formation to heat the oil. The only significant energy loss is the fuel energy used by the air compressor and is about 25% of the total energy consumption.
Another significant difference between the conventional surface steam generator (SSG) and the downhole steam generator (DSG) is the fact that a DSG injects all the gases of combustion into the formation along with the steam, while the SSG exhausts the gases to the atmosphere causing environmental problems. The gases injected amount to approximately 3600 scf per barrel of steam. The gases introduce recovery mechanisms in addition to the viscosity reduction associated with the injection of steam alone. The additional recovery mechanisms include: increased reservoir pressure, movement of oil through viscous drag, reduction of oil viscosity and swelling of the oil through gas solubility.
Furthermore, the injection of gases downhole are known to reduce air quality problems associated with the surface steam generators. Environmental concerns will have a significant impact on oil/gas recovery operations in Canada and the U.S. in the future. However, downhole steam generation techniques appear to meet that challenge as well.
The Department of Energy in the U.S. contracted the Sandia National Laboratories to develop a downhole steam generator. The Sandia Labs team developed a downhole steam generator and field-tested it in Long Beach, Calif.
The Sandia design differs markedly from the process described in U.S. Pat. No. 4,604,988, in that the flame is surrounded by metal walls which are cooled from the outside by circulating water through the sleeves around the combustion chamber. All other designs of DSGs make use of a similar cooling technique. This leaves the metal surfaces of the burner exposed to the high temperature flame on the inside while the outside is cooled by the circulating water. Thus, enormous temperature differences occur across the thickness of these walls resulting in severe thermal stresses. Therefore, the field tests conducted using this design have reported severe metal damage in the flame zone, which resulted in shutdown of the DSG in just 65 hours of operation. In one test the entire burner head had melted away. One of the major recommendations was to lower the combustor wall temperatures by design modifications. The thermal stresses were also responsible for the severe cracking and consequent corrosion of the burner components used in the field tests. These problems are still being tackled, and appear to be the main hurdle for this technology to be accepted by the oil recovery industry.
The process described in the '988 patent does not have this problem of severe thermal stresses in metal walls, which has been the source of major operating problems of other DSG designs. In the described process the flame is surrounded by a rotating body of water, and not by metal walls. Therefore the temperature of the combustion chamber walls is the same as that of the cold water. There is no scope for thermal stresses to develop, and therefore will not result in cracking or corrosion. This is one of the most important advantages of the concept described in the '988 patent over that of other DSG designs. Use of the '988 patent process could insure long and trouble-free operation of the downhole steam generator in the reservoir environment, provided, however, that the process can be successfully adapted to the efficient generation of steam. As noted earlier, however, the process described in the '988 patent is not of sufficient efficiency for generating steam. Such modification of the process described in the '988 patent to allow steam generation, especially downhole, would be of great value to the industry.